The present invention relates to a fiber optic gyro in which a clockwise propagating light ray and a counterclockwise propagating light ray are entered into a loop-shaped optical fiber coil and, by taking advantage that a phase difference is caused between said clockwise and counterclockwise propagating light rays according to an input angular rate applied to the optical fiber coil, the input angular rate is detected More particularly, the invention relates to such a portion that modulates the phases of said clockwise and counterclockwise propagating light rays so as to make a phase difference between said clockwise and counterclockwise propagating light rays alternately take +.pi./2 radian and -.pi./2 radian with an input angular rate of zero, while synchronizing such a phase modulation signal as described above with a light propagation time in the optical fiber coil.
FIG. 1 shows an example of a conventional fiber optic gyro. A light beam emitted from a light source 1 is entered into a beam splitter 13 via a beam splitter 12. The light beam is split into two rays by the beam splitter 13 and entered into an optical transmission path (i.e., an optical fiber coil) 14 as a clockwise propagating light ray and a counterclockwise propagating light ray. At that time, the clockwise propagating light ray is entered into the optical fiber coil 14 via a phase modulator 15. The clockwise light ray leaves the optical fiber coil 14 and is transmitted through a biasing phase modulator 16 and then returns to the beam splitter 13. The counterclockwise light ray is transmitted into the phase modulator 16 from the beam splitter 13, then proceeds to the optical fiber coil 14 and, after that, it enters the beam splitter 13 via the phase modulator 15 where the counterclockwise light ray is combined with the clockwise light ray while causing interference therebetween. The again combined interference light beam is entered into the beam splitter 12 from which the beam propagates to an opto-electric converter 17 where the beam is converted to an electric signal FR representing the intensity of the combined light (interference light).
In a fiber optic gyro, the relative phase difference between the clockwise light and the counterclockwise light, after returning to the beam splitter 13, is 0 unless the optical fiber coil 14 revolves around an axial center thereof. When the optical fiber coil 14 revolves around the axial center thereof, an irreversible phase shift is created between the clockwise light and the counterclockwise light owing to the Sagnac effect while resulting in a change of the intensity of the combined light (interference light) detected by the opto-electric converter 17, because of said phase shift. Said phase difference and the change in the intensity of the combined light (interference light) takes a relationship as shown in FIG. 2 as represented by a sine function. The magnitude and direction of the change in the intensity of said interference light are detected in order to detect an input angular rate. According to the prior art, in order to have the fiber optic gyro actuated at a most sensitive operating point, for example, at the point of a phase difference of .+-..pi./2 radian, a phase modulation signal BPM in a rectangular wave with a period of 2T is generated from a phase modulation signal generator 18. The biasing phase modulator 16 is thereby activated to give phase shifts of .pi./4 radian and -.pi./4 radian alternately in light signals. The time interval T is made equal to a light propagation time T through the optical fiber coil 14. Therefore, as shown in Rows B and C of FIG. 3A, when the clockwise light ray and the counterclockwise light ray return to the beam splitter 13, the phases of both light rays are shifted by .pi./4 radian (CWP) and -.pi./4 radian (CCWP), respectively at the same time. Consequently, the phase difference between both light rays repeatedly becomes .pi./2 radian and -.pi./2 radian in a cycle time of T each, as shown in Row D of FIG. 3A. Obviously in FIG. 2, the sensitivity becomes maximum at phase differences of .pi./2 and -.pi./2 and, at these operating points, the interference light takes the same level of optical intensity. Therefore, the level of an output FR from the opto-electric converter 17 becomes stationary as shown in Row E of FIG. 3A.
The output FR of the opto-electric converter 17 is amplified by an amplifier 19 and the amplified output thereof is synchronously detected by the output of the phase modulation signal generator 18 in a synchronous detector 21. The synchronous detector 21 issues an output signal that shows the magnitude and direction of a phase shift in relation to the bias phase modulation signal BPM. Said output signal is integrated in an integrator 22, and an integrated output thereof is supplied to a ramp voltage generator 23 which issues a ramp signal output having a slope and polarity according to the input thereto. Said ramp signal output activates the phase modulator 15 which generates a phase difference cancelling the phase difference induced due to the Sagnac effect between the clockwise light and the counterclockwise light. At that time, by measuring the frequency and plurality of the ramp voltage signal by a means not illustrated, the magnitude and direction of the input angular rate are obtained.
If the time interval T used to modulate phases is smaller than a propagation time r through the optical fiber coil 14 in the case where an input angular rate is zero, phase modulations CWP and CCWP effected to both light rays returned to the beam splitter 13 take such a relationship as the phase of the counterclockwise light lags by T (.pi. radians) or more from that of the clockwise light, as shown in Rows B and C of FIG. 3B. Therefore, the phase difference PD between these light rays become zero in between .pi./2 radian and -.pi./2 radian. Correspondingly, the output FR of the opto-electric converter 17 becomes a series of pulses as shown in Row E, as revealed from the output-phase difference curve of FIG. 2. When the time interval T of phase modulation is larger than the propagation time .pi., phase modulations CWP and CCWP received by both light rays returned to the beam splitter 13 are such that the counterclockwise light lags by T (.pi. radian) or less from the clockwise light. The phase difference between both light rays becomes zero between .pi./ 2 radian and -.pi./2 radian, in some periods of time where the pulses in the output FR of the opto-electric converter 17 become maximum as shown in Row E.
When the time interval T is not equal to .tau. as described above, pulses larger than the level at a phase difference of .pi./2 radian are generated as an output from the opto-electric converter 17, as if there is an input angular rate which does not actually exist As a result, operation error occurs. Consequently, according to conventional systems, the bias phase modulation signal BPM was generated by the phase modulation signal generator 18 using an output from a voltage controlled oscillator 24. In addition, the output from the amplifier 19 and the bias phase modulation signal BPM of the phase modulation signal generator 18 were supplied to the detector 25. Thus, it was detected whether a pulse in the output FR of the opto-electric converter 17 occurred after or before a rising edge of the phase modulation signal BPM. A detected output controlled the voltage controlled oscillator 24 to make an oscillation frequency of the voltage controlled oscillator 24 lower if said pulse existed thereafter. If said pulse existed before the rising edge, the oscillation frequency of the voltage controlled oscillator 24 was made higher Thereby, it was managed to have no pulse generated from the opto-electric converter 17, that is to make T=.tau..
According to a convention optic gyro as described above, in order to make the bias phase modulation switching time interval T equal to the light propagation time .tau. through the optical fiber coil 14, a compensation was effected by detecting when a pulse generated in the output of the photo electric converter 17 occurred in relation to an edge of the bias phase modulation signal. However, the frequency band of the photoelectric converter 17 is not always wide enough. Therefore, the output pulse of the opto-electric converter 17 may be subjected to waveform distortion or delay, causing an error in it timing relative to the phase modulation signal and making it difficult to precisely compensate the time interval T.